In the field of paint, reduction in use rate of organic solvent in paint has been an important subject, from the viewpoints of environmental preservation and resource saving. As a means to deal with this problem, recently research on waterborne paint and means to increase solid content of paint (high solid paint) for reducing organic solvent content of paint is under progress.
As water-based paint used in the field of paint for automobiles, for example, thermosetting water-based paint comprising polycarboxylic acid resin, amino resin, linear low molecular weight polyester diol and benzoine is known (see e.g., JP Hei 4(1992)-93374A) which, however, has low solid concentration and gives insufficient coating film surface smoothness and unsatisfactory coating film performance such as water resistance.
Of paints for automobiles, particularly top coat paints are required to form coating film of excellent appearance and quality. In metallic coating, it is required to provide coating film of excellent finished appearance such as the metallic texture, by favorably orienting flaky effect pigment such as of aluminum in metallic base paint used.
As a water-based metallic base paint to be used in such metallic coating, for example, a base coat composition which allows the metallic paint therein to exhibit the maximum flip-flop effect and gives excellent metallic texture, characterized by having the water-based base paint contain fine particles of crosslinked polymer (e.g., see JP Hei 3(1991)-14869B). However, the base coat composition has a defect that it tends to develop film defects such as sagging, non-uniformity and the like under change in coating conditions, in particular, humidity change.
Furthermore, in respect of 2-coat-1-bake coating system, also a coating film-forming method using as the water-based metallic base paint, for example, an aqueous dispersion containing fine polymer particles, in particular, polymer particles copolymerized with specific long chain monomer, e.g., stearyl acrylate, stearyl methacrylate or the like, has been proposed (e.g., see JP 2001-104878A). The method, however, is subject to such problems as insufficient coating operability and finished appearance of the coated film, e.g., smoothness and metallic texture.